Circular dichroism (CD) is a phenomenon caused by optical activity (chirality) of molecules and is defined as a difference in absorbance with respect to left and right circularly polarized light. Since spectral information of this circular dichroism reflects a high-order structure of molecules, circular dichroism is particularly suitably applied to, for example, analysis of a high order structure of biologically active substance. For this circular dichroism, a method of irradiating a sample with left and right circularly polarized lights and obtaining a difference in absorbance from an intensity difference of transmitted light is generally used.
In measurement of the circular dichroism, measurement using a so-called modulation method is generally used. In a modulation method, an optical phase modulator such as a photoelastic modulator or a Pockel cell is used as a circular polarization modulator that generates circularly polarized light. However, modulation of the circular polarization modulator in which there is a distortion component is known (see, for example, Patent Literature 1 and Non Patent Literature 1).